1. Field of the Invention
This invention relates to an illumination optical system for use in microscopes.
2. Description of Related Art
As a transmissive illumination optical system for microscopes, a Kohler illumination optical system is known. This optical system comprises, at least, a light source, a collector lens, a field stop, an aperture stop, and a condenser lens system. The image of the light source is projected at the position of the aperture stop by the collector lens, and this projected image of the light source is further projected, as a secondary light source, on the pupil of an objective lens by the condenser lens system. In this way, an object is illuminated. On the other hand, the image of the field stop is projected on a sample surface by the condenser lens system. Thus, according to the Kohler illumination optical system, the field of illumination is definitely determined and unevenness of illumination is lessened.
In ordinary microscopes, the position of the pupil of the objective lens is frequently set at infinity, and thus it is common practice that the illumination optical system is placed to be telecentric (so that a chief ray is parallel to the optical axis). This is because, if not, an eclipse will be caused when the aperture stop is stopped down. Hence, in most cases, the aperture stop is disposed to coincide with the position of the object focal point of the condenser lens system.
Microscopes are used in a relatively wide range of magnifications of 1-100.times.for observation. Since, however, a magnification of 1.times.is entirely different from a magnification of 100.times.in size of the visual field and numerical aperture of the objective lens, it is difficult that a single optical system is made to satisfy conditions for realizing the Kohler illumination optical system at any magnification. Therefore, the illumination optical system is divided into several steps according to magnification so that optical systems varying with magnification are used to satisfy the conditions of the Kohler illumination at any magnification.
As an example, an apparatus disclosed in Japanese Patent Preliminary Publication No. Hei 5-134190 is quoted. This apparatus is provided with an illumination optical system for high magnification including a high-magnification aperture stop and a high-magnification condenser lens system; and an optical system for low magnification including a low-magnification aperture stop and a low-magnification condenser lens system. One of these optical systems is selectively inserted in an illumination path to thereby secure a Kohler illumination optical system which is applicable over a wider magnification range (also, the light source, the collector lens, and the field stop are used at either high or low magnification).
In this illumination optical system, however, the low-magnification condenser lens system is constructed with a first positive lens, a negative lens, and a second positive lens, and the low-magnification aperture stop is placed close to the negative lens. Since the second positive lens for projecting the aperture stop at infinity is composed of lens elements having a relatively simple arrangement, aberration is not completely corrected. Consequently, the problem is encountered that the amount of light in the marginal portion is insufficient or unevenness of illumination is produced.
Referring now to FIGS. 1 and 2, the cause of this problem is explained. Each of these figures shows an arrangement behind the aperture stop of the illumination optical system. Reference numeral 1 represents an aperture stop; 2 a second positive lens of a condenser lens system; and 3 a sample surface. Numeral 4 denotes a maximum image height on the sample surface 3; 5 denotes a middle image height; and symbol F.sub.f denotes an object focal point of the second positive lens 2. The above problem is caused by the fact that the spherical aberration of the second positive lens 2 is not completely corrected. The second positive lens 2 has large negative spherical aberration, and thus, in order to satisfy the condition of the telecentric system in the marginal portion of the visual field (namely, to make a chief ray 6 perpendicular to the sample surface 3), it is only necessary to shift the position of the aperture stop 1 closer to the second positive lens 2 than the position of the object focal point F.sub.f of the second positive lens 2. This arrangement increases an angle .theta. made by the chief ray 6 with an optical axis L.sub.c, and by the cosine law, the amount of light in the marginal portion of the visual field is considerably reduced compared with the case where the aperture stop 1 is located at the position of the object focal point F.sup.f of the second positive lens 2.
As shown in FIG. 2, a chief ray 7 of the middle image height 5 crosses the optical axis L.sub.c between the object focal point F.sub.f of the second positive lens 2 and the aperture stop 1. Thus, if the aperture stop 1 is stopped down to diminish its diameter, an upper marginal ray will be eclipsed by the aperture stop 1 and the amount of light at the middle image height 5 will be decreased. This appears as unevenness of illumination.
Also, if correction for chromatic aberration is incomplete, unevenness of color is produced in addition to the above difficulty. This makes it impossible to use the optical system.
An example of the condenser lens system (for low magnification) used in the prior art illumination optical system is shown in FIG. 3. A conventional condenser lens system 10 includes a first lens unit G.sub.1 consisting of a positive lens L.sub.1 and a negative lens L.sub.2 and a second lens unit G.sub.2 consisting of a positive lens L.sub.3. The aperture stop 2 is placed close to the first lens unit G.sub.1 between the first lens unit G.sub.1 and the second lens unit G.sub.2. Symbol F.sub.f denotes an object focal point of the second lens unit G.sub.2 and F.sub.b denotes a combined image focal point of the first and second lens units G.sub.1 and G.sub.2.
The following is the numerical data of lenses constituting the condenser lens system 10 used in the conventional illumination optical system.
______________________________________ r.sub.1 = 23.83 d.sub.1 = 8.0 n.sub.1 = 1.6779 .nu..sub.1 = 55.33 r.sub.2 = 108.255 d.sub.2 = 16.0 r.sub.3 = -22.599 d.sub.3 = 2.0 n.sub.3 = 1.64769 .nu..sub.3 = 33.8 r.sub.4 = -267.886 d.sub.4 = 30.7 r.sub.5 = 39.982 d.sub.5 = 5.5 n.sub.5 = 1.7725 .nu..sub.5 = 49.66 r.sub.6 = -74.922 d.sub.6 = 9.2 ______________________________________
Focal length f.sub.1 of the first lens unit G.sub.1 =100.026 Focal length f.sub.2 of the second lens unit G.sub.2 =74.9 Focal length f of the condenser lens system 10=67.352 Effective illumination area D of the object surface=22
In the numerical data which is mentioned above and will be described later, r.sub.1, r.sub.2, . . . represent radii of curvature of individual lens surfaces; d.sub.1, d.sub.2, . . . thicknesses of individual lenses or spaces therebetween; n.sub.1, n.sub.2, . . . refractive indices of individual lenses; and .nu..sub.1, .nu..sub.2, . . . Abbe's numbers of individual lenses.
TABLE 1 ______________________________________ Illumination optical system of prior art Theoretical When AS is opened When AS is stopped down value (*1) (*2) ______________________________________ On-axis 100 100 100 Middle 99 92 76 Margin 96 94 78 ______________________________________
In Table 1, the theoretical values that the amount of light on the optical axis is taken as 100 and the amount of light at each image height is taken as zero in aberration, are colapared with the amounts of light of the conventional illumination optical system. The column of the conventional illumination optical system gives a case (*1) where the aperture stop (AS) is set so that the numerical aperture (NA) is 0.08 and another case (*2) where the AS is stopped down so that the NA is 0.04, under the conditions that the NA of the objective lens is 0.04 and the chief ray at the maximum image height is parallel to the optical axis. It is seen from the above data that in the conventional illumination optical system, the amount of light in the marginal portion is materially decreased compared with that on the optical axis, brightness at the middle image height is reduced, and at the same time, unevenness of illumination is produced.